a. Field of the Invention
This invention relates to rate-responsive pacemakers and, more particularly, to pacemakers that employ a respiration sensor as a metabolic rate indication and uses the respiration as a means of controlling their pacing. More particularly, the present invention pertains to a pacemaker wherein a rate responsive parameter is derived from said metabolic rate indication, said parameter being modified synchronously with respiration.
b. Description of the Prior Art
Many attempts have been made to control the heart rate of a pacemaker patient so that it will duplicate the intrinsic heart rate of a healthy person both when the patient is at rest and when the patient is involved in various levels of exercise. Metabolic demand related parameters heretofore proposed for controlling the pacing rate include the QT interval, respiration rate, venous oxygen saturation, stroke volume, venous blood temperature, and minute volume or ventilation, among others. (The terms minute ventilation and minute volume are used interchangeably). In addition, the use of mechanical sensors which detect patient motion have also been explored in such attempts at achieving improved rate-responsiveness.
Of the various parameters available, it has been found that pacemakers using minute volume as a parameter for controlling pacing rate are particularly advantageous.
It has been observed that one of the short term physiological activities which affect the intrinsic heart rates of healthy individuals is respiration. This phenomenon, referred to as Respiratory Sinus Arrhythmia (RSA), is a well known phenomenon which has been described in the literature in 1847. See Saul et al. ASSESSMENT OF AUTOMATIC REGULATION IN CHRONIC CONGESTIVE HEART FAILURE BY HEART RATE SPECTRAL ANALYSIS (Am. J. Cardiol 1988; 61:1292-1299). Sobh et al. ALTERED CARDIORESPIRATORY CONTROL IN PATIENTS WITH SEVERE CONGESTIVE HEART FAILURE: A TRANSFER FUNCTION ANALYSIS APPROACH; IEEE Computers in Cardiology 0276-6547/96, p. 33-36 1996; Hayano et al. RESPIRATORY SINUS ARRHYTHMIA, Circulation, Vol. 94, No. 4, Aug. 15, 1996; Saul et al. NONLINEAR INTERACTIONS BETWEEN RESPIRATION AND HEART RATE: CLASSICAL PHYSIOLOGY OR ENTRAINED NONLINEAR OSCILLATORS; IEEE Computers in Cardiology 0276-6574/89/0000/0299, 1989, p. 299-302.
In normal individuals the heart rate varies in response to autonomic as well as other regulatory inputs to the sinoatrial (SA) node. The highest frequency variations are a result of parasympathetic input and are modulated by respiration. For this reason, heart rate variation in the greater than 0.15 Hz range is commonly referred to as respiratory sinus arrhythmia (RSA). Although referred to as an arrhythmia, this rate variation with respiration has been found to be important to survival (i.e., individuals without RSA have higher rates of overall mortality than those with RSA). A decrease in RSA typically coincides with heart disease, such as congestive heart failure.
Heart rate variability (HRV) due to RSA has been generally thought to be simply a result (i.e., an indicator) of healthy autonomic function with no intrinsic value. A hypothesis is that HRV in and of itself is in some ways beneficial to health and survival. HRV may be antiarrhythmic as some studies have shown a decreased HRV precedes ventricular tachycardia. See Huikuri, et al. FREQUENCY DOMAIN MEASURES OF HEART RATE VARIABILITY BEFORE ONSET OF NON-SUSTAINED AND SUSTAINED VENTRICULAR TACHYCARDIA--Circulation Vol. 87,1804, April 1993, pp. 1220-1228. A paper published recently in Circulation (Circ 1996;94:842-847) showed that RSA vs. fixed pacing rate improves pulmonary gas exchange and circulatory efficiency. However, until now pacemakers have ignored this phenomenon.
In the following description of the invention, it should be understood that rate responsive systems making use of the minute volume as a parameter first calculate a long term average for the minute volume of a patient and then determine the difference between this long term average and an instantaneous minute volume obtained as described below. The resulting differential parameter is referred to as "the minute volume" for the sake of brevity. However, in the drawings, the parameter is identified as dmv to indicate that, in fact, this parameter corresponds to the variation of the instantaneous minute volume from a long term average value.